It is reasonable to begin a study of the neural mechanisms of pruritus with an analysis of how itch is signaled by the peripheral nervous system. It is not known which cutaneous receptor elicit itch. For the most part, classification of the receptors in mammalian skin is complete. Adequate stimulus requirements for many receptors have been documented, and each receptor population has been correlated with a particular cutaneous sensation. However, there is an exception. The sensation of itch has never been associated with a particular receptor population. Since itch is a unique sensory experience, there must be some characteristic mechanism by which it is encoded by the peripheral nervous system. There are two possible alternatives. (1) A receptor population has been missed. (2) There is a population of receptors that responds both to itch-producing stimuli and to stimuli that elicit other sensations. In the present experiments, it is proposed to test these two alternatives by systematically recording single-unit activity from each type of cutaneous sensory neuron and applying itch-producing substances. All known receptor types have been sampled in cat and only polymodal receptors respond. Hence, itch is signalled either by an unknown receptor or by polymodal receptors.